DESCRIPTION (Applicant's Abstract) Physical activity declines markedly in girls during the high school years and the determinants of this decline are poorly understood. The proposed investigation builds on an ongoing NHLBI-funded study for the purpose of expanding the body of knowledge on factors that influence change in physical activity in girls during their high school years. The ongoing study has observed the effects of a comprehensive, school-based intervention on physical activity in high school girls in the 9th grade. The proposed investigation will extend the current study by administering follow-up measures of physical activity and its hypothesized determinants to 12th grade girls who have attended participating schools since the 9th grade. The sample will include a sizeable cohort of girls who were observed in the 8th and 9th grades. The study will address two major aims. Aim 1: To determine the long-term effects of attending a school in which the intervention was implemented during their 9th grade year, approximately 1,825 12th grade girls in 11 intervention and 11 control high schools will complete measures of physical activity, physical fitness, and hypothesized determinants of physical activity. A pooled cohort/cross-sectional design will be employed to compare girls in intervention schools with those in control schools, and the unit of analysis will be the school. Aim 2: To identify factors that mediate and moderate changes in physical activity in girls during their high school years, structural equation modeling will be employed. Second-order factor latent growth curve analysis will be used to test the influence of selected factors on longitudinal changes in outcome variables. These analyses will be performed, with the individual student as the unit of analysis, in the cohort of approximately 1,165 girls who will have been observed in the 8th, 9th, and 12th grades. The analyses will consider the effects of exposure to the intervention, race/ethnicity, and weight status on the psychosocial and environmental factors that are hypothesized to influence change in physical activity over time. The proposed investigation will provide a unique and comprehensive examination of the factors that influence change in physical activity in high school girls.